1. Field of the Invention
This invention generally relates to a non-invasive therapeutic ultrasonic system, and more particularly, to a system which is capable of acoustically imaging and heating a certain region to be treated ("the treatment region") in target tissue for therapeutic purposes as well as acoustically monitoring the temperature profile of the treatment region.
2. Description of the Related Art
The absorption of energy in tissue, for example, in the human body produces an increase in temperature, which can be exploited for therapeutic purposes. The irradiation of ultrasound to the target tissue such as in the human body, which has been successfully used for decades mainly in increasingly sophisticated diagnostic imaging applications, also allows the target tissue to absorb a certain amount of energy. Thus, ultrasound may be used in the therapeutic uses.
Specifically, ultrasonic energy at frequencies above 1.5 MHz has an acoustic wavelength less than 1 mm in the human tissue. This energy is easily controlled in beamwidth and depth of penetration, and has a favorable absorption characteristic in the tissue. These aspects allow the energy to be precisely localized such that regions may be selectively heated while sparing overlying tissue structures.
Ultrasound has significant advantages for therapeutic applications as compared to micro-wave radio-frequency (RF) energy or optical energy (laser light). In contrast with the ultrasound, the RF energy is characterized by long wavelengths in the tissue, with limited to poor control of energy deposition, and high absorption. These aspects of the RF energy constrain its therapeutic usage to large superficial areas. On the other hand, the optical energy which is typically emitted from lasers can be precisely controlled in beamwidth, but the opacity and high absorption in the tissue also limits its use to surface treatment or invasive procedures. Furthermore, the laser and RF energy are emitted from ionizing radiation sources which are typically associated with some risk, unlike acoustic transducers which are typically used for generating ultrasound.
However, in contrast with the diagnostic uses, the therapeutic uses of ultrasound such as hyperthermia and non-invasive surgery have seen relatively little progress due to several technological barriers.
The primary impediment has been a lack of the ability to monitor temperature in the treatment region during the therapeutic treatment process.
Specifically, one of objectives of the therapeutic application is to create a very well-placed thermal gradient in the target tissue to selectively destroy certain regions thereof. For example, hyperthermia technique typically requires to maintain the tissue temperature near about 43 degrees Celsius, while the goal of non-invasive surgery is typically to elevate the tissue temperature above and beyond about 55 degrees Celsius. Moreover, during the therapeutic treatment process, the physiological response of the target tissue is directly related to the spatial extent and temporal duration of the heating pattern. Consequently, in order to appropriately perform feedback and control of the therapeutic treatment process for obtaining successful results, it is absolutely essential to monitor the temperature in the target tissue, for example, so as to know whether or not the temperature in the treatment region has been raised to a level that produces a desired therapeutic effect or destruction in the tissue. In addition, it is preferable to know the temperature distribution in the treatment region and the vicinity thereof for enhancing the therapeutic effect.
In the conventional technique, the therapeutic ultrasonic system has typically relied upon thermocouple probes for monitoring the temperature in the treatment region and the vicinity thereof. However, the thermocouple probes are highly invasive because they have to be inserted into the region-of-interest. In addition, use of the thermocouple probes has necessarily led to very poor spatial resolution since only a small number of probes could be safely embedded in the region-of-interest. Furthermore, the thus embedded thermocouple probes are likely to disturb the acoustic propagation in the tissue and typically cause excessive heating at the probe interface during the therapeutic treatment process. This results in undesirably modified temperature distribution as well as erroneous measurements.
Another factor which has curtailed progress in the therapeutic uses of ultrasound has been the design of the conventional acoustic transducers.
In general, for the therapeutic treatment process, imaging of the treatment region is necessary to determine the location of the treatment region with respect to the acoustic transducers as well as to evaluate progress of the treatment process. Such essential functions of imaging as well as the aforementioned temperature monitoring may be implemented with the same acoustic transducer to be used for the therapeutic purposes, since the acoustic transducers can actually produce an image of the region-of-interest by employing well-established imaging technique such as B-scan imaging. However, the conventional acoustic transducers which are typically employed for the therapeutic purposes are acoustically large, often single-element devices having narrow bandwidth in the frequency domain. Although they are designed to efficiently transmit acoustic energy to the target tissue, the conventional acoustic transducers are typically unsuited for imaging of the treatment region and/or monitoring the temperature profile therein. This precludes development and implementation of these vital functions for performing a desirable precise therapeutic treatment process.
Some prior art references teach the use of ultrasound for therapeutic purposes. For example, U.S. Pat. No. 4,757,820 to Itoh discloses an ultrasound therapy system having functions of imaging and heating the target using ultrasound beams for the therapeutic purposes. The system disclosed therein, however, does not have the temperature monitoring function.
U.S. Pat. No. 5,370,121 to Reichenberger et al. discloses a method and an apparatus for non-invasive measurement of a temperature change in a subject, in particular a living subject, using ultrasound waveforms. The method and apparatus disclosed therein, however, relies on a differential ultrasound image between two successive ultrasound images of the target. In other words, any temperature change is detected as a temperature-induced change in brightness between the two images, which appears in the differential image. Consequently, an actual real-time monitoring of the temperature may be difficult in the disclosed method and apparatus. Moreover, although the method and apparatus can detect changes in the temperature of the target, an absolute value of the target temperature may not be obtained therefrom. In addition, any movement of the target may introduce changes in the differential image, which may cause erroneous results.
Furthermore, although it is not distinctly intended to be applied in the therapeutic treatment process for the target tissue such as in the human body, U.S. Pat. No. 5,360,268 to Hayashi et al. discloses an ultrasonic temperature measuring apparatus in which a temperature of the target medium is calculated using a propagation time of ultrasonic waves which propagated for a predetermined distance in the target medium. The apparatus disclosed therein, however, is mainly described as employing separate ultrasonic elements which respectively function for a transmitter and a receiver of the ultrasonic waves.
While some prior art temperature monitoring techniques exist, see, for example, U.S. Pat. No. 4,807,633 issued to Fry on Feb. 28, 1 989, such techniques are complex and have limited applicability. That is, use of such techniques essentially preclude use of the system for purposes of imaging, unless one were to use multiple transducers. In that regard, while two or more physically separated transducers can be used to accomplish imaging and therapy, typically with one configured for imaging and the other for therapy, such a system is susceptible to the generation of imprecise data and is overly complex and expensive.
Thus, it would be advantageous to provide a compact, non-invasive system capable of acoustically performing the therapeutic heating and the imaging of the treatment region in the target tissue as well as the temperature monitoring in the treatment region with a single acoustic transducer.